1. Field of the Invention
The present invention relates to a multi-tube burner and to a manufacturing method that uses this burner.
2. Description of the Related Art
A vapor phase axial deposition (VAD) method and an outside vapor phase deposition (OVD) method are known as methods for manufacturing glass preforms, in particular, the base material of optical fibers. In these methods, a glass feedstock gas is jetted from a burner together with adjunct gas, flammable gas, combustion aiding gas, and the like, and fine particles of glass (soot) are synthesized by causing the glass feedstock gas to undergo a hydrolysis reaction in a flame. The fine glass particles are accumulated in outer peripheral portions and bottom end portions of a starting member, thereby enabling a porous glass preform to be obtained. This porous glass preform is then sintered in an electric furnace so that it changes into transparent glass which forms the base material for an optical fiber.
Burners used in the above process include a multi-tube burner in which a plurality of gas jet nozzles for the various gases used in the synthesis of the fine glass particles are provided in a concentric configuration, and a multiple nozzle burner in which a plurality of combustion aiding gas jet nozzles are provided between a plurality of flammable gas jet nozzles that are arranged in a concentric configuration. These burners are generally formed from quartz glass in order to avoid contamination from impurities.
However, if a large number of glass preforms are manufactured continuously using a conventional burner, as time passes, contamination of the distal end portion of the burner caused by adhesion thereto of fine glass particles and the like occurs, as does wear of the distal end portion of the burner. If problems in the distal end of the burner such as these arise, the fine glass particle accumulation efficiency is lowered, and glass preforms of a consistent quality cannot be obtained. For example, as the number of manufactured glass preforms rises, the problem arises that the outer diameter of glass preforms obtained under the same manufacturing conditions becomes gradually smaller.
The fine glass particle accumulation efficiency is defined as being the ratio of the total number of fine glass particles accumulated in the starting member relative to the total number of fine glass particles if it is assumed that all of the glass feedstock gas used is changed into fine glass particles by chemical reaction.
If the outer diameter of the glass preforms becomes gradually smaller, then, using the VAD method, it is not possible to obtain the target wavelength dispersion values or the target draw diameter. Moreover, using the OVD method, the target wavelength dispersion values and mode field diameter and the like could not be obtained In addition, if the outer diameter of a porous glass preform becomes gradually smaller, the bulk density thereof changes and there have been breakages in some porous glass preforms. If this type of phenomenon arises, the yield of optical fiber base material ultimately obtained is reduced.
The present invention provides a multi-tube burner that suppresses changes in the outer diameter of a glass preform during the manufacturing of that glass preform and enables a glass preform of consistent quality to be manufactured consecutively, and also to a glass preform manufacturing method that uses this multi-tube burner.